A review was undertaken of the methodology utilized by NIOSH during investigations of building indoor air quality (IAQ). NIOSH investigation teams generally included an industrial hygienist, a doctor, and an epidemiologist, and sometimes included engineers. Investigations usually consisted of building background assessments and on site assessments. The background assessment included the collection of information on worker symptoms and details of the problem being experienced. A questionnaire sometimes has been utilized to facilitate this data collection. Site assessments typically included a conference with the employer and workers in the building in question, a walk through survey, interviews with employers and workers, an assessment of the ventilation and climate control systems, and environmental monitoring of the indoor environment. The problems most commonly identified in NIOSH building investigations were inadequate ventilation in 52%, contaminants from inside the building in 17%, contaminants brought in from outside in 11%, microbial contamination in 5%, and building material contaminants in 3%; in 12%, no problem could be identified.